Tropical Storm Moke is forecast to stay south of Hawaii and to put up to 15 inches on the Big Island, and no tropical cyclone warning is in effect anywhere

Fifteen inches.

That is the maximum the Central Pacific Hurricane Center puts on the Big Island of Hawaii from Tropical Storm Moke, on top of a general 5 to 10 inches across the windward and southeast slopes. Eastern Maui is given 3 to 6 inches. The rest of the islands are given one or two.

The storm itself is not forecast to arrive. Advisory 11, issued at five in the afternoon Hawaii standard time on Saturday, puts the centre at 15.3 north and 152.1 west, about 360 miles south-southeast of Hilo, moving west at 14 mph with maximum sustained winds of 45 mph and a central pressure of 999 millibars. On the forecast track it passes south of the Big Island on Sunday and well south of the smaller main islands early next week. Tropical storm force winds reach 115 miles out from the centre, which does not close the gap.

No warnings, and two other things

There are no coastal watches or warnings in effect. That is the advisory's own line, and taken on its own it would mislead, because the National Weather Service office in Honolulu has two products standing.

A flood watch covers the Big Island from midnight on Saturday night through Monday afternoon. A wind advisory runs until six on Sunday evening for parts of Maui, Molokai, Lanai, Kahoolawe and the Big Island, for east-northeast winds of 25 to 35 mph gusting to 55, strongest over ridges and gaps and where the wind comes down off high ground.

The tropical cyclone warning system and the local one are separate products with separate triggers. On this storm only the second has anything to say.

Why the ground matters more than the total

Because it is already full.

The flood watch is blunt about it. It will not take a significant amount of rain to cause flash flooding, the text says, because the ground is saturated from recent heavy rain. The hurricane center's advisory names the same ground from the other direction, singling out areas affected recently by Lala.

Lala is not a memory. The Central Pacific Hurricane Center is running advisories on it at the same time as on Moke, and at five on Saturday afternoon it was still a tropical storm carrying 70 mph winds, weakening, north of the Papahanaumokuakea Marine National Monument. This publication covered its hurricane warning for Hawaii County and Maui on 14 August.

What the forecast admits about itself

Moke is at or near its peak. Dry air and vertical wind shear are expected to start weakening it on Sunday, and the outlook has it post-tropical by Thursday and gone by Friday. Underneath the numbers the forecast advisory prints its own accuracy record, as those documents do: track errors have averaged near 150 nautical miles on day four and 200 on day five, and intensity errors near 20 knots on every day of the forecast.

None of that reaches the next 48 hours. Those are the ones with the rain in them.